1. Field of the Invention
The present invention relates to a CDMA mobile communications apparatus and a base station detecting method used for the apparatus, and more specifically to a procedure (cell search) for use by the CDMA (Code Division Multiple Access) mobile communications apparatus detecting a base station which is a direct connection partner.
2. Description of the Related Art
Conventionally, in the CDMA mobile communications, in order to synchronize the CDMA mobile communications apparatus (hereinafter referred to as a mobile unit) with the base station, there are a system and a sub-system of using three physical channels for constant transmission from the base station, that is, pSCH (Primary Synchronization Channel) and the sSCH (Secondary Synchronization Channel), and the CPICH (Common Pilot Channel).
These systems are described in the Standard Specifications TS25.211, TS25.213, and TS25.214 by the 3GPP (3rd Generation Partnership Project) which is a standard specification association of the CDMA mobile communications.
The structure of the physical channel shown in the standard specification TS25.211 has a radio frame of 10 ms as a basic unit, and one frame is formed by 15 slots as shown in FIG. 5. The symbol rate of the physical channels pSCH, sSCH, and CPICH is 10 per slot, but the pSCH and sSCH are assigned only to the leading symbol period of a slot.
The pSCH is spread by a spread code PSC (Primary Synchronization Code) commonly used in all base stations. The sSCH is spread by 16 types of SSC (Secondary Synchronization Code) assigned to each slot according to the unique sequence corresponding to 64 code groups to which the Scrambling Code assigned to each base station described later belongs.
The CPICH is spread by the Scrambling Code assigned to each base station. The other physical channels excluding the pSCH and sSCH transmitted from a base station are spread by the same Scrambling Code, and these physical channels are spread by a chanelization code assigned to each physical channel. However, the explanation is omitted here.
A common base station detecting method using the above mentioned three physical channels is described in the standard specification TS25.214. A base station detecting process is performed by detecting a frame transmission timing of a base station with a chip precision which is a spread unit, and by further detecting a Scrambling Code which is used at the base station. The process is performed in three steps of ‘slot synchronization’, ‘frame synchronization/code group identification’, and ‘Scrambling Code identification’.
These steps will be briefly explained below. First, in the ‘slot synchronization’ step (hereinafter referred to as step #1), in the timing of all chips in 1 slot period, a correlation operation corresponding to the spread code PSC is performed, and the profile of the correlation power value for the timing is obtained. When the pSCH transmission timing of the base station matches the timing of the correlation operation of the mobile unit, a large correlation value appears in the profile.
Commonly, a digital matched filter having the spread code PSC as a filter coefficient is used in the correlation operation. When the reception base band signal of the mobile unit is input into the matched filter, a sharp peak is output at the timing of the pSCH matching the filter coefficient. By detecting the peak, the synchronization is obtained with the transmission timing of the pSCH, that is, the slot timing.
In the ‘frame synchronization/code group identification’ step (hereinafter referred to as step #2), at the synchronous slot timing in step #1, correlation operations of 16 types are performed. Each SSC correlation value is obtained over a plurality of slots, the correlation values are accumulated according to 64 SSC sequences of the SSC, and the code group is identified in order from the highest accumulation value. Furthermore, a frame can be synchronized from the sequence of the SSC.
In the ‘Scrambling Code identification’ step (hereinafter referred to as step #3), at the frame timing synchronizing in step #2, a correlation operation for 8 Scrambling Codes belonging to the code group identified in step #2 is performed, and the Scrambling Code is identified in order from the highest correlation value.
When transmission signals of a plurality of base stations are received by the mobile unit at different timing, the correlation value profile obtained in step #1 shows a plurality of peaks corresponding to the timing and the received power. Therefore, on each of the plurality of peaks obtained in step #1, the processes in steps #2 and #3 are repeatedly performed, thereby detecting a plurality of base stations.
If a plurality of base stations are detected by this method, either the processing method shown in FIG. 6 or the processing method shown in FIG. 8 can be used to select a plurality of peak values from the correlation value profile obtained in step #1.
In the case of the processing method shown in FIG. 6, the peak value in the correlation value profile having a larger correlation power value is selected (steps S21 to S22 in FIG. 6), and the processes in and after step #2 are sequentially performed. For example, when the correlation value profile as shown in profile as shown in FIG. 7 is obtained, then the processes in and after step #2 are performed in the order of (a)→(b) →(c)→(d)→(e)→(f).
When the processing method shown in FIG. 8 is performed, a threshold T1 is set for the power value of the correlation value profile, the correlation power value equal to or smaller than the threshold T1 is processed as noise (steps S31 and S32 shown in FIG. 8), the correlation power value larger than the threshold T1 is selected in the order of the occurrence of a peak value (steps S31, S33, and S34 shown in step S8), and the processes in and after step #2 are sequentially performed. For example, when the correlation value profile as shown in FIG. 9 is obtained, then the processes in and after step #2 are performed in the order of (a)→(b)→(c)→(d)→(e)→(f).
In the above mentioned conventional CDMA mobile communications, the environment as shown in FIG. 10 is assumed. If a mobile unit (mobile object provided with a mobile unit) 201 is moved in the right direction in FIG. 10, and if a shelter 301 exists between a base station 103 and the mobile unit 201, then the electric wave from base stations 101 and 102 can be predicted and therefore transmitted or received by the mobile unit 201 with sufficiently strong power. However, the waves from the base station 103 are reduced by the shelter 301 and received by the mobile unit 201 with lower power than the base stations 101 and 102.
When the mobile unit 201 starts detecting the base stations 101 to 103 in the above mentioned method in the state shown in FIG. 10, then the order in which the base stations 101 to 103 are detected is not certain in the processing method shown in FIG. 8. However, in the processing method shown in FIG. 6, the probability that the base station 103 can be detected is clearly lower.
Furthermore, in the state shown in FIG. 10, if the mobile unit 201 starts communications with the base station 101, some time has passed, and if the state shown in FIG. 11 is entered, no influence of the shelter 301 is detected any more. When the most appropriate base station to the mobile unit 201 is the base station 103 as a connection partner, the mobile unit 201 performs hand-over from the base station 101 to the base station 103.
However, if the mobile unit 201 cannot detect the base station 103 in the order of detecting base stations, it cannot perform the hand-over, and the electric wave from the base station 101 becomes weaker, thereby disconnecting the communications between the mobile unit 201 and the base station 101.